Method and means of selective removal of background indications in stabilized water-washable inspection penetrant processes

ABSTRACT

An improvement in the water-washable inspection penetrant process wherein a &#39;&#39;&#39;&#39;stabilized&#39;&#39;&#39;&#39; background-producing waterwashable inspection penetrant is employed and the water-wash step is modified to include a brief spray-wash with water, application of an accelerator rinse of a low energy detergent solution, and a final water rinse, whereby background indications of shallow surface discontinuities are selectively removed to a controlled degree.

United States Patent [1 1 Alburger Nov. 6, 1973 [54] METHOD AND MEANS OFSELECTIVE 3,429,826 2/1969 Alburger 250/71 T REMOVAL OF BACKGROUND2,806,959 9/1957 DeForest et al 250/71 T 3,456,110 7/1969 Diperstein250/71 T INDICATIONS IN STABILIZED 35587882 1/197 [76] Inventor: JamesR. Alburger, 5007 Hillard Ave., La Canada, Calif. 91011 22 Filed: Mar.24, 1972 ABSTRACT [21] Ap 1. No.: 237,765 An im rovement in thewater-washable ins ection en- P P P P etrant process wherein astabilized" backgroundproducing water-washable inspection penetrant isem- 250! zifi gg 'z ployed and the water-wash step is modified toinclude '5 Fie'ld T. 73/104 a brief spray-wash with water, applicationof an accel- 232/301 erator rinse of a low energy detergent solution,and a final water rinse, whereby background indications of I 56] R f res Cited, shallow surface discontinuities are selectively removed e e toa controlled degree. I UNlTED STATES PATENTS 3,422,670 1/1969 Alburger73/104 3 Claims, 2 Drawing Figures WATER-WASHABLE INSPECTION PENETRANTPROCESSES Fijalkowski 250/71 T Primary Examiner-Harold A. Dixon A L LPENE/MA/TS 1 51 51 7 5A/5/7'/I// 7V WASH W475? GOA 7457' 77/1/15" M/AU7Z'5 METHOD AND MEANS OF SELECTIVE REMOVAL OF BACKGROUND INDICATIONS INSTABILIZED WATER-WASHABLE INSPECTION PENETRANT PROCESSES RELATED PATENTSU.S. Pat. No. 3,107,298, Apparatus for the Measurement of FluorescentTracer Sensitivity.

U.S. Pat. No. 3,164,006, Evaluation Performance of Liquid PenetrantTracer Materials.

U.S. 282,843, No. 3,282,843, Emulsifier Compositions.

U.S. Pat. No. 3,31 1,479, Penetrant Inspection Process and Compositions.

U.S. Pat No. 3,349,041, Gel-Forming Inspection Penetrant and EmulsifierCompositions and Processes.

U.S. Pat. No. 3,422,670, Cleaning Process and Compositions forPost-Emulsifier Inspection Penetrants.

U.S. Pat. No. 3,429,826, Gel-Forming Inspection Penetrant and EmulsifierCompositions Employing Hydrophylic and Lipophylic Surfactants."

U.S. Pat. No. 3,527,709, Fluorescent Tracer Process and Compositions.

U.S. Pat. NO. 3,557,015, Dual-Sensitivity Dyed Liquid Tracers.

U.S. Pat. No. Re. 26,888, Process for Fluorescence Detection ofExtremely Thin Tracer Films.

The inspection penetrant process is used extensively for the detectionof potential failure flaws in machine parts, such as jet engine turbineblades, bearings, and the like. In Essence, the process involves thesteps of applying an oily, dyed penetrant liquid to test parts, by dip,brush, or spray; cleaning off surface penetrant by emulsification andwashing; and inspecting the .test parts for traces of penetrant liquidwhich remain trapped in flaws open to the surface. In some cases, adeveloper, such as a fine, fluffy powder or a suspension of powderparticles in a liquid, may be applied to test surfaces just prior toinspection, so as to draw entrapments of penetrant out of surfacecracks, thereby enhuncing the detectability of such flaw indications.

Modern inspection penetrants utilize fluorescent dyes of various kindsfor the purpose of providing high sensitivity in the detection ofsurface flaws, and the inspection of parts thus tested is usuallycarried out in a semi-darkened booth, and under ultraviolet irradiationprovided by a suitable black light lamp.

Two kinds of inspection penetrant processes are in vogue, one beingknown as the P/E or post-emulsifier process, the other being known asthe W/W or waterwashable process. In the P/E process, the penetrant isan oily, water-insoluble liquid containing a dissolved indicator dye (ordyes). After application of this oily liquid to test surfaces, thesurfaces are drained and an in the case of extremely shallow crackdefects, porosity defects, and inter-granular corrosion defects.

It is possible to reduce the effect of emulsifier stripping of flawentrapments in the P/E process by reducing the activity of theemulsifier, in accordance with the teachings of my U.S. Pat. No.3,422,670, or by increasing the resistance of the penetrant toemulsification, in accordance with the teachings of my U.S. Pat. No.3,311,479. However, in the former case, such low energy emulsifiers mustbe applied by spray, and this mode of usage is not always convenient ordesirable. In the latter case, there is a limit to the ability of knownliquid material compositions to resist emulsification and at the sametime remain practical for usage as inspection penetrant vehicles.

Much of the military and commercial usage of penetrants is nowconcentrated on the W/W process, for the reason that water-washablepenetrants are simple to use, requiring only the two basic steps ofpenetration and water-wash removal. Also, processing costs are minimizedby the elimination of emulsifier inventories and equipment such asemulsifier dip tanks and related apparatus.

The W/W process also suffers from the inherent problem of leaching outof flaw indications during the step of wash-removal of surfacepenetrant. Wash water tends to diffuse into entrapments of penetrant insur face flaws, causing them to wash out to a greater or lesser extent,depending on the time duration of wash water contact with thetestsurface.

A very significant improvement in the ability of penetrant entrapmentsto remain in surface flaws during processing has been achieved bycausing the penetrant liquid to form a viscous gel upon contact withwater, as is taught in my U.S. Pat. Nos. 3,282,843, 3,349,041, and3,429,826. It is even possible to provide an improved retention of flawentrapments (or indications) simply by utilizing a penetrant liquidwhich has a relatively high viscosity. For example, many penetrantliquids have viscosity values (rated at 100 F.) on the order of 3 to 5cst. Thus, a W/W penetrant vehicle having a viscosity which is highrelative to 3 or 5 cst., as, for example, 15 to 50.cst., will exhibit anoticeable improvement in retention of indications during wash removalof penetrant. As already mentioned (supra), the use of a gel-forming W/Wpenetrant provides, a very pronounced enhancement of indicationretention, particularly if the penetrant liquid is designed in such away that it passes through a gel-condition upon contact with wash water,providing a rise in viscosity of the penetrant/water mixture to a valyeabove to cst.

The degree of indication retention may be further stabilized by use ofspecial sensitizer dyes, as is described and claimed in my copendingapplication, Ser. No. 222011, filed Mar. 13, 1972, for Method' and Meansof Enhancing and Controlling Retention of Flaw Indications inWater-Washable Inspection Penetrant Processes.

The ability of an inspection penetrant to reveal the presence ofextremely small defects in surfaces depends in part on the dimensionalthreshold of fluorescence (or color) response in the indicator dye whichis present in the penetrant, and this dimensional sensitivitycharacteristic may be conveniently evaluated by use of the so-calledMeniscus-Method" in accordance with the teachings ofmy U.S. Pat. No.3,107,298. Thus, if surface flaws were to retain entrapments ofpenetrant throughout the entire penetrant process without any loss ofleaching out, then the dimensional threshold of fluorescence or colorwould be a true index of the flawdetecting capability or sensitivity ofthe penetrant. Since leaching out of indications always does occur inpractice, the extent or degree of such leaching out must be taken intoaccount in the determination of the flaw detection sensivitity of a W/Wpenetrant.

I have devised instrumentation and measurement techniques for accuratelyevaluating the indication retention capability of a given penetrant as afunction of remover contact time, be the remover an emulsifier or washwater, and I have discovered a new and novel method and means ofproviding a very substantial improvement in the stability of W/Wpenetrant indications and their ability to resist wash removal, thismethod and means being described and claimed in my above-mentionedcopending application, Ser. No. 222011. In fact, the improvement whichmay be effected in indication retention is so dramatic that mystabilizer method and means cannot normally be used to its fullestextent in accordance with conventional wash-removal techniques, sinceunder such conditions a tremendous amount of fine surface-structuredetail is revealed, and this tends to interfere with the detection andevaluation of potential failure defects in highly stressed parts. Thus,for most practical inspection application, where conventional washingtechniques are utilized, it has been found desirable to employ only apartial stabilization of indication retention. Even at such conditionsof partial stabilization, the use of high levels of sensitivity in thepentrants induces the formation of an excessive degree of backgroundindications of fine surface porosity features.

In any event, W/W inspection penetrants may be stabilized by variousmeans and to varying degrees of indication retention. in all cases, asthe degree of stabilization increases, and as the level of sensitivityincreases, the problem of unwanted background indications becomes morepronounced.

For purposes of the present invention, the term stabilization ofindication retention refers to any of the above-mentioned methods ofenhancing the retention of indications, and stabilized W/W penetrantsare any of those W/W inspection penetrants in which retention ofindications is enhanced by means of increased viscosity, the feature ofgel formation, or by use of special high-stability sensitizer dyes. v

l have discovered that it is possible to utilize a stabilized W/Wpenetrant at a degree of stabilization and at a sensitivity level whichwould normally yield an excessive amount of background indications ofshallow surface discontinuities, and by suitable modification of thewashing step, the unwanted background indications may be selectivelyremoved without loss of actual crack indications.

The principal object of the invention, therefore, is to provide a methodand means of selectively removing unwanted back-ground indications in astabilized W/W inspection penetrant process.

This and other objects of the invention will in part be obvious and willin part become apparent from the following description thereof whenreadin conjunction with the drawings in which:

FIG. 1 is a graphical illustration of a family of characteristic curvesfor different conditions of indication retention in W/W inspectionpenetrants, and

FIG. 2 is a graphical illustration ofa family of fluorescence transitioncurves for various W/W inspection penetrants having different levels ofdimensional sensitivity.

In order to understand properly the scope and significance of thepresent invention, it is first necessary to discuss a technique formeasuring the degradation (or retention) of indications as a function ofremover contact time. A suitable method and instrumentation for thispurpose is summarized as follows: 7

Aluminum test panels are prepared in accordance with the teachings of myU.S. Pat. NO. 3,164,006 by anodizing a sheet of type 1 100.0 aluminum,0.050 inch thick. The aluminum is anodized in a sulfuric acid bath to ananodic film thickness of 0.001 inch, and the anodic coating isheat-sealed in boiling water. The thusanodized aluminum is cut intopanels about 4 X 4 inch and the individual panels are further treatedwith a surface sealant in accordance with the teachings of my copendingapplication, Ser. No. 237984, filed Mar. 24, 1972, for Improved CrackedTest Panel for Evaluation of Inspection Penetrant Performance. Thethusprepared panels are bent over a cylindrical form having a radius ofcurvature of about 0.75 inch, and are then flattened out. The bendingoperation is conducted in two directions, at 90 to each other, whereby amass of fine closely spaced craze cracks are produced in the center ofthe test panel. Alternatively, or as a supplement to the bendingoperation, the aluminum test panel may be heated on a hot plate to about400 F. so as to create or further augment the craze cracking of thebrittle anodic coating.

A meter suitable for measurement of fluorescent brightness response isconstructed using a photomultiplier tube as the photosensor, and thephototube is arranged so as to plug into a microscope tube with a 10xobjective lens. In this way, the photomultiplier tube sees a small areaon the microscope stage about onesixteenth inch in diameter. This areaneed not be in accurate focus. Stops are mounted on the microscope stagein such a way that the aluminum test panel may be placed on the stagerepetitively in exactly the same position. Thus, each time the testpanel is placed on the stage, the photocell sees exactly the same areaof the craze-cracked panel. The photomultiplier instrument is exactlylinear in its response, so that initial brightness readout conditionsmay be established, providing a full- 7 scale meter reading of 100.Thereafter, subsequent readings of diminished brightness may be read outas percentages of the initial brightness value simply by noting themeter reading.

In use, the W/W penetrant being tested is applied to the crazed surfaceof the test panel, and excess penetrant is wiped off by means of papertowelling. The test panel is then polished with a soft tissue so as toentirely remove surface penetrant, leaving only penetrant which isactually trapped in the craze cracks. These penetrant entrapments willfluoresce and may be seen readily or read out by means of the photocellsensor, for the reason that the anodic coating on the test panel iscompletely transparent in character.

The thus-prepared panel is placed on the microscope stage, and the blacklight lamp position and multiplier voltage conditions are adjusted so asto provide a fullscale meter reading of 100. The panel is then removedfrom the microscope stage and is immersed in water for a pre-selectedtime interval which may range from a few seconds up to or minutes. Afterexpiration of the period of water-contact, the panel is air-dried undera jet of compressed air, and a brightness measurement is again taken,using the standardized photomultiplier setup.

By repeating the above-described procedure, using different durations ofwash-water contact, it is possible to plot curves of indicationdegradation as a function of wash-water contact time. In a similarmanner, it is possible to plot curves of indication degradation for aP/E penetrant as a function of emulsifier contact time.

I have found that any interruption of water contact, during thewater-immersion of a W/W penetrant, such that the surface film of wateris replaced by air, even mementarily, may disturb the interfacialtension conditions present in the penetrant entrapments, thus making thetest results unreliable. If, however, the panel is kept fully immersedin water, test measurements of indication degradation may be reproducedaccurately, within a probable error of only a few percent.

Another device which is useful for evaluating penetrant performance is ametal panel which has been sandblasted to a standard degree of surfaceroughness. A suitable panel may be prepared from one-eighth inch thickannealed type 301, or 302, stainless steel 2 X 4 inches in size. Thepanel is sandblasted on one side with 100 mesh average size grit, using60 pounds of air presthe evaluation of the amount of backgroundindications which remain after washing'the penetrant-treated surfacewith water. In simplest terms, the degree of background retention on arough surface for a given penetrant after processing may be rated inaccordance with the fluorescent brightness which is seen under standardconditions of black light irradiation. Alternatively, the brightness ofthe test surface may be measured as a percentage of the brightness of astandard fluorescent plaque under a given intensity of ultravioletirradiation. Still another measuring procedure and associated instrumentwhich are important in connection with the present invention are theso-called Meniscus Method and the Meniscus Method Apparatus, thesehaving been disclosed and claimed in my U.S. Pat. NO. 3,107,298. TheMeniscus-Method instrument consists essentially of a black glassoptically polished platen and a clear glass convex lens ground as, asurface radius of 106 cm. When the convex lens is placed on the flatplaten, and a few drops of fluorescent dyed liquid is placed between thelens and platen, and the assembly is irradiated with ultraviolet light,then a nonfluorescent spot appears in the meniscus-shaped layer ofliquid around the point of contact between lens and platen. The diameterof this spot is measured asthe distance'between points of half themaximum brightness of the fluorescent specimen. For the purpose of thisspecification, all references to spot diameter" shall be taken to meanthe distance between half-brightness points as measured by the MeniscusMethod.

The feature of indication retention for various types of penetrantcompositions is illustrated in FIG. 1. For

the purpose of consistency in this illustration, all of the penetrantformulations tested were adjusted so as to provide the same dimensionalthreshold of fluorescence, as measured by the Meniscus Method, and asrepresented typically by a spot diameter of 3.0 mm. Similar curves areobtained when the test formulations are adjusted so as to providedifferent spot diameters, which, as mentioned above, may range fromsomewhat less than 1.5 mm. to more than 15 mm.

Referring now to FIG. 1, there is here illustrated a family of curvesshowing the indication retention behaviour of various penetrantcompositions. In this figure, the percentage of retained indicationbrightness is plotted on the axis of ordinates against wash-watercontact time plotted on the axis of abscissas. Curve 21 illustrates thedegradation of indication brightness for a penetrant compositioncontaining only a conventionaltype sensitizer dye, and in which thepenetrant liquid is a simple water-soluble solvent such as diethyleneglycol.

Curve 22 shows the feature ofindication degradation where the sensitizerdye is a conventional coumarin dye, and the penetrant liquid is aself-emulsifiable, but non-gel-forming composition.

Curve 23 shows the feature of indication degradation where thesensitizer dye is a conventional coumarin dye, and where the penetrantliquid is a gel-forming composition of the type disclosed and claimed inmy U.S. Pat. No. 3,282,843.

Curves 24 and 25 illustrate the feature of indication degradation (orretention) for gel-forming penetrants having stabilization factors of 50percent and percent, respectively, in accordance with the teachings ofmy above-mentioned copending application, Ser. NO. 222,01 1. Y

I have discovered that all presently known inspection penetrantmaterials are characterized by flaw entrapment efficiencies which arerelatively low. Actually, the

flaw entrapment efficiency of a penetrant is difficult to evaluateaccurately, since the geometry of a crack in the test panel is verydifficult to analyze. Even though the depth of the crack may beaccurately controlled, to 40 microns for example, the width of the crackis considerably smaller, being some indeterminatefunction of theintercrystalline separations which occur in the anodic film during thebending and cracking operation. Thus, the volume of penetrant liquidactually retained in a given crack behaves as though. it has anequivalent film thickness considerably smaller than 40 microns.

Referring now to FIG. 2, there is here illustrated a family offluorescence response transition curves, 1 to 10, representing various standardlevels of penetrant sensitivity which are employed in industry. Curve 1,for example, corresponds to a level 1 penetrant which exhibits aMeniscus Method spot diameter of 8.5 mm. Curve 7 exhibits a spotdiameter of 3.0 mm., and so on. Line 11, to the right in FIG. 2,represents the depth (40 microns) of a standard crack in anodic coatingas described. It is seen that the dimension of 40 microns is large withrespect to the film thickness at which typical fluorescent penetrantsundergo transition of fluorescence response. However, due to the factthat the volume of penetrant which may be retained as an entrapment inthe crack is quite small (due to the narrow nature of the crack), theequivalent film thickness of a given entrapment is considerably lessthan 40 microns. I have found that equivalent film thickness values forresidual entrapments of penetrant (after a specified wash-water contacttime) may be expressed in terms of the relative values of residualfluorescent brightness.

Line 12 in FIG. 2 is a locus line which indicates the relativebrightness levels of indications of a standard 40 micron depth crackpattern which are obtainable with fully stabilized W/W penetrants ofvarious sensitivity levels. For example, when a level 1 fully stabilizedpenetrant is utilized on the 40 micron depth crack pattern, the relativebrightness of the indication pattern, after one minute wash-watercontact time, is about 25 percent as indicated by point 13. If the samecrack pattern is tested using a level 7 fully stabilized penetrant, therelative brightness of the indications is about 53 percent, as indicatedby point 14 in FIG. 2.

The features pertaining to relative indication brightness, which areillustrated in FIG. 2, are applicable to fully stabilized W/Wpenetrants, and the effective brightness conditions of an indicationpattern is the maximum which can be obtained with fully stabilizedpenetrants. It will be understood that partially stabilized penetrantsor nonstabilized penetrants will exhibit less effective brightness ofindications, depending on the degree of stabilization which pertains.

Unfortunately, when the indication retention stability and/orsensitivity of a W/W penetrant are made sufficiently great to revealsmall cracks, the penetrant also acts to yieldindications of minutesurface discontinuities. For example, even a low sensitivity penetrant(level 1), corresponding to curve 1 in FIG. 2, will, when utilized at ahigh degree of indication retention, reveal surface discontinuities assmall or smaller than 0.5 micron in depth, corresponding to point 15 inFIG. 2. At this point 15, the relative brightness of such indications isabout 0.04 (4 percent), and a mass of such indications can provide anobjectionable background luminosity which will interfere with the visualdetection of slightly larger crack defects.

1 have discovered that it is possible to selectively eliminate unwantedbackground indications of shallow surface discontinuities in testsurfaces without loss of actual crack indications. First, though, it isnecessary to start with a condition of indication retention such thatthe desired crack indications are sufficiently stable that they remainat a reasonable level of fluorescent brightness throughout-the varioussteps of penetrant application, wash removal of surface penetrant, andinspection.

To accomplish this end (of eliminating unwanted background indications),the wash-removal step is modified so as to introduce a step of applyingan accelerator rinse to the test surface. The accelerator rinse may beany oil/water emulsifier or any water-soluble detergent which has beendiluted to provide a relatively low' Emulsifier Activity Quotient.

In my U.S. Pat. No. 3,422,670, I have disclosed a method and means ofreducing the activity, or energy, of oil/water emulsifiers by dilutingthe emulsifier or detergent composition to an appropriate degree.Conventional emulsifiers or full-strength detergent mixtures usuallyhave Activity Quotient values in excess of about 0.3. Low energyemulsifier or diluted detergent compositions suitable for use in theprocess of the present invention may have Activity Quotient valuesbetween about 0.000] and 0.2.

The operation of the modified wash removal step of the invention isillustrated by the following example.

A jet engine turbine blade having a heat-resistant coating was immersedin a high-stability penetrant having the following composition:

No. 2 Absportion Oll (Chevon) 25 gal. Oll-soluble Alkanolamide (Emcol 51l) 5 gal. 10 mol Ethoxylated Nonylphenol 20 gal. Diethylene GlycolMonobutyl Ether 5 gal. C.l. B.A. 68 7.9 lb. Fluoranthene 21 lb. C.l.Solvent Yellow 43 2 lb.

test surface. The low energy detergent (accelerator rinse) wasformulated as follows:

9-mol ethoxylated nonvlophenol 8 parts diethylene glycol 2 parts water200 parts In the above detergent formula, a wide varie ty ofwater-soluble detergents may be used in place of the 9-mol ethoxylatednonylphenol. Ethylene oxide mol ratios may vary from about 6 to 30, andthe alkyl chain length may vary from 8 to l2 or more. Different chemicalstructures of the water-soluble detergent may be utilized, such as thosedescribed in my US. Pat. NO. 3,429,826. Also, many water-emulsifiablecompositions, such as those disclosed in my US. Pat. Nos. 3,282,843,3,349,041, and 3,429,826, may be employed under appropriate conditionsof dilution.

The diethylene glycol employed in the above formulation is used merelyas a gel-breaker, so as to facilitate dilution of the detergent inwater. This ingredient may be omitted without any loss in performance ofthe diluted mixture. The degree of dilution may be varied from about 10parts water (one to one) up to as much as 20,000 parts water (2,000 to1), depending on the mode of accelerator usage.

As low dilution ratios, the accelerator rinse acts to a degree as acontact emulsifier, and the length of contact time on the acceleratorrinse with the test surface must be kept quite short, such as a fewseconds; otherwise, desired crack indications may be removed along withbackground porosity indications. With dilution ratios of about 20 to l(200 parts water in the formulation), accelerator rinse contact times onthe order of 10 to 30 seconds may be utilized without excessive removalof actualcrack indications. When the dilution ratio is made very great,in the range of 200 to l or more, the activity of the accelerator rinsebecomes so low that the simple contact with the rinse composition doesnot act to remove background indications rapidly enough for practicalusage. In such cases, the action of the accelerator rinse may be mademore effective by employing a continuous spray application onto testsurfaces. In the example given (20 to l dilution ratio), the acceleratorrinse may be applied by spray, brush, dip, or in any suitable manner, solong as the test surface is completely wetted by the rinse composition.

In any event, the application of the accelerator rinse onto thepre-washed test surface serves to alter the detergent chemistry of thepenetrant entrapments so that entrapments in shallow discontinuitiestend to wash off of the surface readily, while entrapments in deepcracks remain stable and non-washable. If it were possible to drain thetest surface sufficiently, after application of the penetrant, so thatan entirely uniform coating of penetrant remains on the test surface,then the pre-wash step using a spray of water could be omitted, and theaccelerator rinse could be applied directly onto the penetrant-coatedpart. However, considerable variations occur in the thickness of thepenetrant coating, and this serves to prevent uniform action of theaccelerator rinse in the selective removal of background indications.For most practical purposes, it is desirable that a pre-wash be carriedout prior to application of the accelerator rinse, so as to present arelatively clean test surface to the action of the accelerator rinse.

In the present example, following the application of the acceleratorrinse, the test part was again rinsed briefly under a spary of water.Equivalent results are obtainable with a simple dip-rinse in water.Excess water was blown off the test part by means of compressed air, andthe part was then dried and inspected under black light. It was observedthat several crack indications were revealed against a background whichhad an extremely low level of fluorescence.

The ability of the foregoing process to reveal crack indications withoutsubstantial loss of fluorescent brightness was tested as follows: Theexact same procedure as outlined above was carried out on a crackedanodic panel having surface cracks 40 microns deep. A similar processwas carried out on a companion cracked anodic panel with 40 micron depthcracks, except that in this latter case, the step of application ofaccelerator rinse was omitted. Examination of the two cracked panelsunder black light showed that the crack indications produced weresubstantially equivalent as to brightness.

The same two processes (with and without accelerator rinse) were carriedout on two rough sandblasted stainless steel panels, and examination onthe washed and dried panels under black light showed that the processwhichincluded the accelerator rinse produced virtually no backgroundindications; whereas, when the accelerator rinse was omitted, backgroundindications of roughness were so excessive asto be unacceptable forpractical inspection usage.

In the above penetrant formulation, the ratio of equivalentconcentration of the C.l.- BA. 68 and the Fluoranthene may be varied inaccordance with the teachings of my copending application, Ser. No.222,01 1, so as to provide a desired degree of sensitizer v dyestabilization. Also, the total amount of sensitizer dye present may bevaried so as to provide a desired level of dimensional sensitivity, asis'well known in the art.

With regard to the C.I.- Solvent Yellow 43 constituent, it is normalpractice in the formulation of inspection penetrants to utilize acolor-former dye, such asa yellow fluorescent dye, for the purpose ofshifting the color of fluorescence from the characteristic blue emissionofthe sensitizer dye to a yellow or green hue which is close to the peakof spectral sensitivity of the human eye. In some cases, it may be founddesirable to employ a dye having a visible color, such as blue or red,so as' to provide a dual-sensitivity feature in accordance with theteachings of my U.S. Pat. No. 3,557,015. Thus, any

one of a wide variety of supplemental color-former dyes may be employedin the formulation, and the con-' any W/W penetrant, Ihave found thatdistinctly beneficial and advantageous results are obtained when thepenetrant is a stabilized or partially stabilized composition formulatedin accordance with the teachings of my copending application. Ser. No.222,011.

In every fluorescent inspection penetrant composition, thecharacteristic dimensional threshold of fluorescence response depends onthe specific sensitivity of the fluorescent sensitizer dye which isemployed, and its concentration. Many suitable sensitizer dyes aredisclosed in my U.S. Pat. No. Re. 26,888, but for use in any oilypenetrant vehicle, regardless of whether it is a P/E or W/W material,the sensitizer dye must be selected with regard to its solubility in theoily liquid vehicle. Among the better fluorescent dyes, at least withregard to solubility in oily penetrant vehicles, are various coumarinderivatives, and various heterocyclic fluores cent dyes.

A few of the suitable fluorescent dyes which may be utilized as thenonstabilized element of the sensitizer component of the penetrantcompositions may be identified as simple 7-coumarin compounds, a fewexamples being:

4-methyl-7-amino-coumarin 4-methyl-7-'ethylaminc-coumarin4-methy1-7-dimethylamino-coumarin 4-methyl-7-diethylamino-coumarin4-methyl-7-(ethyl-benzylamino)-coumarin3-benzyl-4-methyl-7-ethylamino-coumarin3-benzyl-4,6-dimethyl-7-ethylamino-coumarin3-benzyl-4-methy1-7-amino-coumarin3-benzyl-4-methyl-7-dimethylamino-coumarin 3-phenyl-7-aminocoumarins Inaddition to the foregoing, various other coumarin deriviatives may beemployed, such as those which are described in U.S. Pat. No. 2,88l,l86,these being 3-substituted-aminocoumarins;.or those which are describedin U.S. Pat. No. 2,929,822, these being 3-phenyl-7-carbalkoxy-amino-coumarin compounds, or those which aredescribed in U.S. Pat. No. 2,945,033, these being various7-triazinylamino-3-phenylcoumarins. Also, certain heterocyclic compoundsmay be employed, similar to those which are described in U.S. Pat.No.2,875,089, these being compounds containing azole or imidazole'ringstructures. A number of suitable sensitizer dyes may be drawn from thelisting of Fluorescent Brightening Agents as set forth in the ColorIndex, published by the Society of Dyers and Colourists, Dean House,Picadilly-Bradford, Yorkshire,

England. These include, in part, C.l. Fluorescent Brightening AgentsNos. 8,9, 36, 53, 57, 58, 60,61, 68,

69, 72, 78, and 130.

In the above listing, several of the brightening agents arecoumarin-type dyes. For example, Brightening Agent No. 68 is a coumarindye, probably 4-methyl-7- diethylamino-coumarin. This is a commonlyavailable dye, supplied by several dye manufacturers under thedesignation Color Index Fluorescent Brightening Agent No. 68. Forpurposes of this specification, the above designation is abbreviated toC.I.- B.A. 68."

Conventional ordinary dyes, typified by those listed above, all providea non-stabilized character for indications derived from water-washablepenetrants. For purposes of this specification, the degree of indicationretention for such oridinary sensitizer dyes is about 25 percent for awash water contact duration of l minute, and penetrants containing onlysuch sensitizer dyes are said to have zero" stabilization. The rating of25 percent indication retention is, of course, a function of the crackdepth in the crazed-crack test panel, and of the basic physicalproperties of the penetrant vehicle; viscosity, gel-forming features,etc., and this rating for the so-called non-stabilized condition may,therefore, vary somewhat from one penetrant formulation to another.

Among all of the many hundreds of fluorescent dyes which I have tested,I have discovered one type substance, and-one only, which exhibits afeature of extremely high flaw indication retention. This substance isthe material fluoranthene (including derivitives of fluoranthene).

The use of fluoranthene as a penetrant sensitizer has been disclosed andclaimed in my US. Pat. Nos. Re. 26,888 and 3,527,709. However, I havemade the new discovery that fluoranthene may be substituted in acontrolled manner, in part, for a conventional sensitizer such as acoumarin-type sensitizer mentioned above, in such a way that the featureof indication retention is enhanced to a predictable and controllabledegree. This phenomenon is observed only when the fluoranthene materialis utilized in a W/W-type penetrant. The effect of enhancement ofindication retention does not occur when the fluoranthene substance issubstituted for a conventional sensitizer in a P/E penetrant.

l have utilized fluoranthene as a penetrant sensitizer, in the past, inconnection with the formulation of P/E- type penetrants, and such usagesare mentioned in my U.S. Pat. Nos. 3,311,479 and Re. 26,888. I have eventried using fluoranthene as the (sole) sensitizer dye in W/W penetrantformulations, but have found such W/W penetrant compositions to beunacceptable for commercial inspection usage. Accordingly, up to thepresent time, I have avoided the use of fluoranthene in W/W penetrantcompositions, mainly because I have considered fluoranthene to becontra-indicated for such usage due to its almost complete insolubilityin water. Now, however, I have discovered that new and novel results ofcontrolled partial stabilization of indication retention may be achievedby combining fluoranthene with a conventional sensitizer dye inappropriate proportional amounts in W/W penetrant formulations.

For example, the use of a conventional (coumarintype) sensitizer dyealone (with no fluoranthene), provides a dye stabilization factor ofzero. As fluoranthene is substituted for the conventional dye, the dyestabilization factor increases, being stated as the equivalentpercentage of fluoranthene contained in the total amount of sensitizerdye which is utilized.

Equivalent percentages of fluoranthene refer to the effective"quantities of fluoranthene, and not to the actual quantities present asmight be measured on a weight basis. This is because a conventionalcoumarintype dye, such as C.I.- BA. 68 is from 2.5 to 2.8 times moreeffective in its specific sensitivity than is fluoranthene. Thus, forfluoranthene to be present in an equivalent percentage equal to thecontent of CI.-

BA. 68, corresponding to a stabilization factor of 50 percent, theactual content of fluoranthene, by weight, must be about 2.5 to 2.8times greater than the weight of the C.l.- BA. 68. This is trueregardless of the actual amount (concentration) of the C.I.- BA. 68which is utilized. Accordingly, the amounts of conventional" sensitizerdye and fluoranthene which may be present in a penetrant formulation arereferred to as proportional equivalent amounts, and the total amount ofboth conventional sensitizer dye and fluoranthene present is referred toas the total equivalent amount of fluorescent sensitizer component.

Thus, the stabilized element of the sensitizer component used in thepreferred W/W penetrant compositions used in the process of theinvention is fluoranthene or certain derivitives of fluoranthene.Fluoranthene may be represented by the empirical formula C l-I or by thestructural formula:

The simple compound, fluoranthene, may be employed, or if preferredcertain fluorescent derivitives of fluoranthene may be employed,suitable substances being 3-fluoro-fluoranthene or 3-amino-fluoranthene.Nonfluorescent derivitives of fluoranthene, such as 3-nitro-fluoranthene, do not function as stabilizers for W/W penetrants.Fluoranthene, itself, is relatively inexpensive, compared withderivitives of fluoranthene, and is, therefore, the preferred materialfor use in the compositions of the invention.

The maximum degree of indication retention is obtained when thesensitizer component of a W/W penetrant consists entirely offluoranthene. Under such conditions, the penetrant may be said to bepercent stabilized, and such penetrants, at level 7 sensitivity, providean indication retention of about 53 percent for a wash-water contactduration of 1 minute. As in the case of conventional sensitizer dyes,some variation in the degree of indication retention may result fromvariations in the physical properties of the penetrant composition.

I have discovered that two mechanisms of indication depletion operate inparallel during the washing step in the W/W inspection penetrantprocess. The first mechanism is one in which a portion of the penetrantentrapment in a surface discontinuity is physically washed away leavinga certain volume of residual penetrant. The second mechanism is one inwhich the indicator dye present in the penetrant liquid is depleted by adiffusion process or a solution process, whereby it becomes leached outto a greater or lesser extent from the residual entrapment of penetrant.

Thus, the first mechanism is essentially mechanical in nature, andindication depletion by virtue of this mechanism may be minimized byemploying high viscosity penetrants or gel-forming penetrants which actto resist the mechanical action of wash removal. The second mechanism isessentially physical-chemical in nature, and indication depletion byvirtue of this mechanism may be minimized by employing a suitableindicator dye which is characterized by extremely low water solubility.

The first mechanism, of mechanical wash-removal of emulsified penetrant,operates in the P/E inspection penetrant process. However, the secondmechanism does not, for the reason that the oily water-insolublepenetrant used in the P/E process does not permit diffusion of water anda resultant leaching action. It would appear that the conventional P/Einspection penetrant process, employing an oily water-insolublepenetrant, and emulsifier, and a water wash, could be made to provideresults which are identical with those obtainable in a W/W inspectionpenetrant process which employs a fully stabilized (insoluble dye)penetrant and a water wash. I have found this to be true, but I havealso found that the PIE process is extremely critical with respect toemulsifier energy and emulsifier contact time. If a low energyemulsifier is utilized, then indication depletion is mimimized, butbackground indications are produced to an excessive degree. And ifasupplemental step of stripping out background indications is introduced,the P/E system becomes a four-step process, resulting in excessive costsdue to added materials and labor.

Contrasted with this, a properly stabilized W/W inspection penetrantsystem is essentially a two-step process. And if a supplemental step ofaccelerator rinse is introduced, in accordance with the presentinvention,

the W/W system becomes a three-step process which provides advantages ofindependent easy control over the rates of indication depletion andremoval of background indications. In addition, material costs arerelativcly low, because of penetrants haing relatively low sensitivity,and greatly diluted detergent materials.

It will be understood, therefore, that defect indications may be formedand/or retained to a desired degree in any one of three ways. First, thewashing step may be made relatively short, so as to minimize the degreeof mechanical removal of penetrant entrapments. Second, the stability ofthe indicator dye may beadjusted to provide a desired rate of dyedepletion from the residual entrapment. Third, the sensitivity level ofthe penetrant may be adjusted by adjusting indicator dye concentration,to a point where the combined action of mechanical wash-removal andindicator dye depletion yields residual fluorescence response of adesired magnitude.

I have found that if it is desired to reveal crack defects which areabout 40 microns deep, a fully stabilized gel-forming W/W penetrant willprovide indications of such cracks to a relative brightness response offrom about 25 percent to about 70 percent (of initial brightness),depending on the dimensional sensitivity ofthe penetrant, as can be seenby points 13 and 16 in FIG. 2. Likewise, a nonstabilized penetranthaving level 7 sensitivity, as illustrated by the transition curve 7 inFIG. 2, would provide indications of 40-micron-depth cracks to arelative brightness response of about 25 percent, as indicated by point18 in FIG. 2, and a nonstabilized penetrant having level 1 sensitivitywould provide a relative brightness for such indications of about 12percent, as indicated by point 17 in FIG. 2. Arrows l9 and illustratehow indications 13 and 14 are degraded from a stabilized" condition todiminished brightness values represented by points 17 and 18,respectively, when nonstabilized-format penetrants are employed insteadof stabilized-format penetrants.

Accordingly, the term stabilized penetrant as used herein refers to aW/W inspection penetrant which is capable of yielding indications of 40micron deep cracks, after 1 minute contact with wash water, to arelative brightness greater than about 12 percent. In any event, whethera high degree of flaw detection sensitivity is derived from a high levelof dimensional sensitivity or from a high degree of indication retentionefficiency, the ability of a stabilized penetrant to display residualindications from 40-micr'on-depth cracks to a relative brightnessgreater than about 12 percent also results in an ability to yieldbackground porosity indications to an objectionable degree.

a The process of the invention, in which the wash step is modified bythe introduction of an accelerator rinse step, may be employed with anywater-washable in spection penetrant, but is utilized to best advantagein connection with stabilized penetrants as identified above.

It will be understood that the process of the invention providesadvantages of accurate control of the amount of residual backgroundindications of fine surface porosity, a minimum number of processingsteps, and economy in mateials used by virtue of the fact that lowcost,low-sensitivity penetrants are operated at high levels of flawentrapment efficiency and low-cost highly diluted low-energy acceleratorrinse solutions are employed.

It will be understood that the accelerator rinse composition of theinvention may be utilized at a wide range of dilution ratios, dependingin part on the strength or activity of the detergent material or theemulsifier which is employed. In the case of the 9-mol ethoxylatednonylphenol material of the example given (supra), a preferred range ofconcentrations is from about 10 to 1 to about to 1.

Although the invention has been described with respect to specificembodiments thereof, it will be understood that various changes andmodifications may be made without departing from the spirit and scope ofthe appended claims.

I claim:

1. A water-washable inspection penetrant process for detecting surfaceflaws in test parts comprising essential steps as follows; (1) applyinga water-washable fluorescent penetrant composition to a test surface,(2) washing off surface penetrant with water, leaving entrapments offluorescent penetrant in flaws open to the surface, (3) applying alow-energy accelerator rinse solution to said test surface for acontrolled period of time ranging from about 10 seconds up to about 10minutes, and (4) inspecting said test surface under black light'for thepresence of said cntrapments of penetrant, said low-energy acceleratorrinse having an activity quotient falling within the range of about0.0001 to 0.2.

2. A process in accordance with claim 1, in which said low-energyaccelerator rinse consists essentially of 9-mol ethoxylated nonylphenoldiluted with about one part to 2,000 parts water.

3. A process in accordance with claim 1, in which said low-energyaccelerator rinse consists essentially of 9-mol ethoxylated nonylphenoldiluted with about 20 parts water.

2. A process in accordance with claim 1, in which said low-energyaccelerator rinse consists essentially of 9-mol ethoxylated nonylphenoldiluted with about one part to 2,000 parts water.
 3. A process inaccordance with claim 1, in which said low-energy accelerator rinseconsists essentially of 9-mol ethoxylated nonylphenol diluted with about20 parts water.